These studies are designed to provide information regarding the physiological regulation of the pituitary-gonadal axis. The axis is subject to two categories of control mechanisms: (1) Internal controls primarily include the various feedback relationships between pituitary gonadotropins and gonadal steroids; (2) External (environmental) controls which, among a variety of factors, include such variables as light (daylength) and temperature. Pituitary-gonadal feedback mechanisms will be studied via an approach designed to yield highly quantitative information under conditions as close as possible to physiological. Continuous intravenous infusions of gonadal steroids will be administered to gonadectomized rats; also, LH and FSH infusions will be administered to hypophysectomized female rats. Serum levels of steroids and gonadotropins will be determined by radioimmunoassay and ovarian follicular development will also be monitored. The main areas of investigation will be: (1) The role of specific gonadal steroids in the regulation of LH and FSH secretion and (2) The specific roles of LH and FSH in the control of follicular development (number of follicles maturing during each ovulatory cycle) and steroid secretion. The effects of photoperiod and temperature on reproduction will be investigated in the Syrian hamster. Emphasis will be on regulation of gonadotropin secretion and on the mechanism of pineal involvement in this seasonally breeding species. BIBLIOGRAPHIC REFERENCES: Goldman, B.D., R.S. Bridges and D.M. Quadagno, 1975. Sexual differentiation and patterns of sexual and maternal behavior in the rat: Role of neonatal gonadotropins and gonadal steroids. In Sexual Behavior: Pharmacology and Biochemistry. M. Sandler and G.L. Gessa (eds), Raven Press, New York, pp. 219-226. Hutchison, J.S. and B.D. Goldman, Intravenous infusion of testosterone in castrated rats: Relationship between rate of infusion and gonadotropin secretion, Endocrinology 97:725-730, 1975.